Southeast Asia is accustomed to intense smoke and haze from forest burning, but in June 2013 air pollution reached the highest level ever recorded -- hitting life-threatening levels in Singapore, for instance. This alone was scary enough, but there is an even more frightening side to the story.

Scary new fire dynamic... (photo by William Laurance)

Unlike virtually all previous mega-fire events in the region, the June 2013 fires didn't occur during a drought. That is unprecedented.

Even in a year with normal rains, fires -- especially in central Sumatra, where forests are being devastated for oil palm, pulpwood plantations, and slash-and-burn farming -- raged out of control.

New research by David Gaveau and colleagues -- which you can download free here -- has uncovered an alarming explanation for this new fire dynamic.

Previous degradation and burning of forests is making them hyper-vulnerable to new fires, even during relatively wet conditions.

Gaveau surveyed the aftermath of the Sumatra burning and found that much of it was caused by slash-and-burn farmers. And most of the burned land was degraded forest or peat-swamp that had already been burned once before.

Gaveau calls these degraded areas "forest cemeteries" -- places where damaged, regenerating forests become prone to a final, fiery death -- and spewing out massive quantities of greenhouse gases in the process.